The interacting effects of genetic variation, habitat quality and population size on performance of Succisa pratensis

1 We studied the performance of 17 Dutch populations of the perennial Succisa pratensis , in relation to population size, genetic variation and habitat quality. We used a path-analytical model to analyse the possible relationships between these variables and performance. 2 Plants in smaller populations produced fewer seeds per flower head. Their seeds had lower germination rates and higher seedling mortality, and more seeds were dormant or non-viable. 3 Population size was also correlated with genetic measures. Small populations had higher inbreeding coefficients than large populations and observed heterozygosity was positively correlated with population size. The mean genetic diversity (expected heterozygosity) was relatively high (H(exp) = 0.42), but not correlated with population size. 4 Less eutrophic habitats appeared to support larger populations. High concentrations of NH(4) and NO(3) in the soil were significantly negatively correlated with population size. 5 Path-analysis showed that Succisa pratensis is vulnerable to habitat deterioration (eutrophication). Population size was strongly influenced by habitat quality. Reduced performance, however, was better explained by direct genetic effects and by habitat deterioration rather than by effects of population size per se . Both habitat quality and genetic effects are thus important for population persistence, even in the short term. The results suggest that there will be a continuing decline of the small populations, due to deteriorating habitat conditions, decreased genetic variation and a reduced reproductive capacity.